KIEV/SLAVIANSK, Ukraine: An agreement reached last week to avert wider conflict in Ukraine was faltering as the new week began, with pro-Moscow separatist gunmen showing no sign of surrendering government buildings they have seized.

Washington says it will hold Moscow responsible and impose new economic sanctions if the separatists do not clear out of government buildings they have occupied across swathes of eastern Ukraine over the past two weeks.

Vice President Joe Biden hopes to demonstrate the US commitment to Ukraine by meeting with its leaders as they deal with their confrontation with Russia.

Biden arrived Monday for a two-day visit to the capital, Kiev. He has meetings Tuesday with the acting Ukrainian prime minister and president. He also is scheduled to meet with legislators and democracy activists before returning to Washington Tuesday night.

He plans to announce new technical support to the Ukrainian government to implement energy and economic reforms.

Despite a new agreement aimed at easing tensions, a shootout Sunday at a checkpoint in eastern Ukraine left at least three dead and Ukrainian and Russian officials trading accusations of blame.

Kiev and Moscow traded accusations over a deadly shooting on Easter Sunday morning, when at least three people were killed at a checkpoint manned by armed separatists. Moscow and its separatist allies accused Ukrainian nationalists of attacking the checkpoint; Kiev said Russia had provoked the violence.

In a later incident, the Ukrainian defense ministry said gunmen on motorcycles fired on an army checkpoint between Donetsk and Slaviansk shortly after dark on Sunday. The troops opened fire, wounding one attacker and capturing two, it said.

Russia, Ukraine, the European Union and the United States signed off on an agreement in Geneva on Thursday, designed to lower tension in the worst confrontation between Russia and the West since the Cold War.

The agreement calls for occupied buildings to be vacated under the auspices of envoys from the Organization for Security and Cooperation in Europe, a security body. All sides are meant to refrain from force.

But no sooner had the accord been signed than both sides accused the other of breaking it, while the pro-Moscow rebels said the pledge to withdraw from occupied buildings was not binding on them.

“Steps are being taken — above all by those who seized power in Kiev — not only that do not fulfil, but that crudely violate the Geneva agreement,” Russia’s

Russian Foreign Minister Sergei Lavrov on Monday accused the government in Kiev of breaching an international accord reached last week aimed at defusing the crisis in Ukraine.

“The Geneva accord is not only not being fulfilled, but steps are being taken, primarily by those who seized power in Kiev, that are grossly breaching the agreements reached in Geneva,” Lavrov said at a televised news conference in Moscow.